1. Field of the Invention
The present invention relates to a stream distribution system that distributes video, audio and other bitstreams stored in a stream server device.
2. Related Art
In recent years, as a result of the rapid expansion of high-speed networks, personal computers (hereafter, “PCs”), digital television receivers and the like into common households, request-type information provider services such as VOD (video on demand) are now becoming widely available.
A request-type information provider service is realized by a stream distribution system that distributes bitstreams of video data, audio data and the like, prestored in a server device or similar center, in response to a distribution request from a user connected to the server device, from the server device to the user that made the request.
FIG. 1 is a structural diagram of a stream distribution system that provides a conventional request-type information provider service.
This stream distribution system is connected to a stream server device 101 owned by the owner of contents such as video, music and the like, and terminal devices 102, 103, 104, . . . , used by respective users, via a network 105 such as the Internet or CATV (cable television). Here, terminal device 102 and the like are pieces of information equipment such as a PC, a set-top box (hereafter, “STB”), a mobile information terminal device (hereafter “personal digital assistant” or “PDA”), and a mobile telephone.
The following description relates to a request-type information provider service in this stream distribution system.
Video, audio and other contents that have been compressed and formed into digital packets (i.e. bitstreams) based, for example, on the MPEG (Moving Picture Expert Group) 2 standard are stored in a storage unit 106 of stream server device 101.
A user using terminal device 102 selects a content from the content bitstreams stored in storage unit 106 of stream server device 101 owned by a content holder, in accordance with a Web browser or similar display mounted in terminal device 102, and presets the desired time of distribution. The name of the selected content, the preset distribution time and other information is transmitted to a control unit 107 in stream server device 101 via network 105.
Having received a request, control unit 107 in stream server device 101 selects a bitstream of the content requested for distribution by the user from storage unit 106 when the preset distribution time is reached, and transmits, via network 105, the selected bitstream to terminal device 102 used by the user that made the distribution request.
As such, it is possible for the user of terminal 102 to view requested contents at desired times.
However, the following problems arise in relation to this conventional stream distribution system due to network traffic congestion and the like caused by the rapidly increasing number of terminal devices connected to networks in recent years.
One significant problem is the increased load on stream server device 101 following increases in the number of users requesting distribution at any one time, and this causes delays and the like due to the distribution functions of control unit 107 in stream server device 101 not working properly. In view of current technical benchmarks, the limit for a single stream server device 101 on a commercial basis is distribution to several thousand terminal devices at any one time, and simultaneous distribution to a greater number of terminal devices than this proves very difficult.
A second problem is that the routes from stream server device 101 to terminals 102, 103, 104, . . . pass through various server devices, routers and the like, this being a characteristic of the Internet, which is presently the primary form of network 105. Use of these routes is not guaranteed at the time of access, and as a result realtime playback in accordance with a user's distribution request it is often impossible.
When these two problems overlap, as in the case of a user making a distribution request of stream server device 101, for example, within a time zone in which distribution requests from other terminal devices are concentrated, or for a bitstream of a content with respect to which there is a concentration of distribution requests, so-called “timeouts”, in which the bitstream is interrupted during playback, may frequently occur.
To date, various techniques have been suggested in order to overcome these problems. In the case of contents being stored in bitstream form based on MPEG-1 or MPEG-2 standards, unexamined Japanese application publication no. 10-336625 discloses, when there has been a distribution presetting from terminal device 102, a technique for distributing, out of a GOP (group of pictures) forming a content whose distribution has been preset, only B-pictures for bi-directional interframe estimation and P-pictures for backward interframe estimation (i.e. I-pictures that have undergone intraframe encoding are excluded) from stream server device 101 to a storage unit 108 provided in terminal device 102 prior to a preset distribution time, and transmitting only I-pictures from stream server device 101 to terminal device 102 when the preset distribution time (realtime) is reached. According to this technique, since bitstreams consisting only of I-pictures need be transmitted realtime from stream server device 101 to terminal device 102, data volume for realtime transmission can be reduced when the proportion of I-pictures in a GOP is low, and as mentioned above, it is thus possible to suppress time zones in which distribution requests are concentrated, suppress increases in network traffic when popular contents are preset for distribution, and suppress load increases on stream server device 101.
According to the above technique, a bitstream of B-pictures and P-pictures, which has been distributed from stream server device 101 without I-pictures, is stored in storage unit 108 in terminal devices 102, 103, 104, . . . , a bitstream of I-pictures distributed from stream server device 101 at a preset distribution time is combined with the bitstream of B-pictures and P-pictures in a RAM 109, which is a volatile random access memory provided in terminal device 102 and the like, and the resultant bitstream is played. Since I-pictures resulting from encoding the original image are not stored in terminal device 102 and so forth, the original image cannot be discerned by a user, even if images formed only from B-pictures and P-pictures which have undergone motion estimation are played.
In other words, since a user cannot decode and view original images prior to a preset distribution time (i.e. until I-pictures are distributed from stream server device 101), this technique makes it possible to distribute bitstreams of part of contents to terminal device 102 and the like in advance without causing concern to content holders, and thus allows also for security protection and copyright protection of contents.
However, this technique gives rise to the following problems.
First, when a large proportion of I-pictures occupy a GOP in a content bitstream, it is preferable not to reduce the data volume of an I-picture bitstream for transmitting from stream server device 101 at a preset distribution time.
Furthermore, in recent VOD technologies and stream distribution systems, bitstream formats for describing contents have shifted from encoding formats standardized in MPEG-1 and MPEG-2, which occupied the conventional mainstream, to bitstream formats standardized by MPEG-4 and having greater compression rates (i.e. greater reduction of the amount of compressed data with respect to the same original data volume).
Because, in the MPEG-4 format, bitstream format is regulated as a prerequisite to transmission/reception via a network, a large number of server devices, routers and so forth at a distribution time, error resilience when via a network is greatly improved in comparison to conventional MPEG-1 or MPEG-2 bitstream formats.
Adapting technology disclosed in the above unexamined Japanese application publication no. 10-336625 to the above MPEG-4 bitstream format allows bitstreams from which the number of I-VOPs (“video object planes”; equivalent of I-pictures in MPEG-1 & 2 formats) has been reduced to be distributed in advance. Because of the high error resilience, measures such as intraframe encoding some of the P-VOPs and B-VOPs (equivalent of P-pictures and B-pictures in MPEG-1 & 2 formats) are taken with MPEG-4 format bitstreams, and thus images that allow estimation of original images can be played in a terminal device, even from bitstreams containing no I-VOPs. This creates significant security problems in relation to copyrights held by content holders (i.e. contents are divulged).
Here, realtime playback of contents is conventionally conducted by transmitting content bitstreams using broadband transmission routes, and in terms of the current state of technology, realtime playback of contents using narrowband transmission routes compares unfavorably to the use of broadband transmission routes in terms of image quality, image size, and the like.